Certain types of fire protection nozzles are used to discharge water, with or without additives, in a relatively fine spray, which is generally referred to in the industry as mist.
The mechanism by which a fine spray (water mist) nozzle system acts to control, suppress or extinguish a fire can be a complex combination of two or more of the following factors, depending on the class(es) of the combustible materials involved, the operating concept of the individual nozzle, the size of the orifice(s), the operating pressure and the flow rate:
(1) Heat extraction from the fire as water is converted into vapor and the fuel is cooled;
(2) Reduced oxygen levels as the water vapor displaces oxygen near the seat of the fire;
(3) Dilution of flammable vapors by the entrainment of water vapor to such an extent that the resultant mixture of vapors will not burn;
(4) Enveloping of the protected area to pre-wet adjacent combustibles, cool gases and other fuels in the area and block the transfer of radiant heat to adjacent combustibles; and/or
(5) Direct impingement wetting and cooling of the combustibles.
In the case of Class A combustibles, a combination of factors (1), (2), (4) and/or (5) may be involved, while a combination of factors (1), (2) and/or (3) may be involved in the case of Class B fires. In order to prevent the electrical conductivity of water from representing a potential problem, the extinguishment of Class C fires by fine water mist is generally limited to nozzle systems which primarily depend on factors (1) and/or (2) only.
It is generally acknowledged that in order for water spray to be described as mist-like, the majority of the water droplets should have a diameter of less than 500 microns (0.020 inch).
However, in the case of Class B fires, the majority of the droplets should have a diameter of less than 300 microns (0.012 inch) in order to maximize the effects of factors (1), (2) and/or (3). In the case of Class C fires, the majority of the water droplets should have a diameter less than 200 microns (0.008 inch) in order to maximize the effects of factors (1) and/or (2) at the smallest practical fire size. In the case of Class A fires, the mist-like droplets may be intentionally combined with a small percentage of high momentum large droplets, in the order of up to 1500 microns (0.060 inch), which serve to entrain and drag the mist-like droplets into the combustion zone, as well as provide some direct impingement wetting and cooling of the combustibles.
Various types of nozzles discharging a fine water spray have long been used in fire protection systems. Although often not described as such at the time, perforated diffuser sprinklers, e.g. as described in Parmalee U.S. Pat. No. 6,275, discharged water in a fine spray by nature of the diffuser holes being in the order of 0.06 inch in diameter. Other examples of fine spray nozzle designs intended for use in fire protection system applications are described in Lewis U.S. Pat. No. 2,310,798, which is based on the use of impinging jets to create a "cloud" of spray, as well as Loepsinger U.S. Pat. No. 2,361,144 and Papavergos U.S. Pat. No. 4,989,675, which are based on establishing a gas-water mixture to create an atomized spray. Further techniques for delivering fine spray for fire suppression purposes include: using an array of nozzles originally designed for fine oil mist atomizing, e.g. in oil burner applications, and using nozzles with an internal fixed scroll, or a whirling device, e.g. as described in PCT Publication No. WO 92/20454.
Within the water spray fire protection field, there has been extensive use of hemispherical or spherical surfaces downstream of the nozzle or sprinkler orifice, to act as a first stage splitter or diverter in conjunction with a second stage deflector which distributes the water spray over the area to be protected. In most of these cases, the splitter is utilized to spread out the stream of fluid flow over a greater area, so that a larger deflector can be used to distribute the fluid over the area to be protected. Such an approach allows a wide range of second stage deflector designs to be implemented for control of the distribution of fluid over the area to be protected. Examples of such use with hemispherical splitter surfaces are given in W. Johnson U.S. Pat. No. 4,465,141 and K. Johnson U.S. Pat. No. 4,596,289. A similar principle is involved in Hanson U.S. Pat. No. 3,051,397, although, in this case, a spherical splitter is used to first fan out the fluid stream against the interior of a cylindrical wall for the purpose of agitating the fluid and entraining air drawn in from the inlet, prior to the resultant fluid mixture being distributed by a downstream deflector, over the area to be protected. However, it should be noted that in this patent, Hanson '397 also indicates that a spherical splitter was used for the sake of simplicity and that a hemispherical splitter would have performed the required function.
There has also been extensive use of hemispherical elements in fire protection nozzles and sprinklers as a mounting location for the fluid deflectors at the junction point of structural support arms extending from the nozzle or sprinkler base. However, in cases such as those illustrated in Martin U.S. Pat. No. 891,278 and Whitaker U.S. Pat. No. 4,585,069, the hemispherical design has also been selected to suitably spread the fluid stream being emitted from the nozzle (sprinkler) orifice over the second stage nozzle (sprinkler) deflector, for distribution over the area to be protected. However, it is well known in the art that, because of the diverting effect that even hemispherical surfaces have on the fluid stream being discharged from the nozzle orifice, their use as a first stage splitter results in a relatively hollow cone or zone of light spray in the region to be protected that is downstream of the nozzle and coaxial with the nozzle orifice. The volume of this zone of light spray may be either increased or decreased by the second stage deflector, depending upon its design. Hanson U.S. Pat. No. 3,051,397 teaches the use of a spherical splitter only for the purpose of simplicity, with acknowledgement that the desired diverting of the fluid stream and the fanning out of the spray to the inside wall of an enclosing cylinder could be performed with a hemispherical splitter. In addition, as illustrated, Hanson '397 required a deflector downstream of the splitter to distribute the fluid mixture over the area to be protected and, the central conical region immediately upstream of the deflector resulted in a zone of light spray in the region to be protected downstream of and coaxial with the device. Furthermore, the spherical splitter is described by Hanson '397 as being selected to have a diameter slightly greater than that of the orifice only to ensure that substantially all of the fluid stream would impinge on the splitter, even if the stream expanded somewhat between the nozzle orifice and the splitter.